Copy protection relates to modifying a video signal in such a manner that a conventional television receiver produces a normal picture from the modified signal, whereas a videotape recording made from the modified signal exhibits very annoying interference.
In the NTSC, PAL, and SECAM standard color television systems color information is transmitted on a subcarrier signal. In this case of the NTSC system the subcarrier has a frequency of about 3.58 MHz and in the case of the PAL system a frequency of about 4.43 MHz. In both systems the precise color at any point in the picture is determined by the phase of this subcarrier relative to some reference phase, while the degree of saturation of the color is determined by the amplitude of the subcarrier.
In order for a television receiver to correctly reproduce colors, the receiver requires information concerning the above-mentioned reference phase. This information is transmitted as part of the video signal in the form of a burst of about nine cycles of subcarrier following the horizontal synchronizing pulse, and is referred to as a color burst. This color burst signal is used in the television receiver to phase-lock a crystal oscillator, thus generating a continuous subcarrier signal at the reference phase which is then used to demodulate the color information. It is normal for the phase-locked crystal oscillator in the television receiver to have a fairly long time constant, on the order of a few milliseconds. The long time constant insures that the oscillator will ignore short term phase perturbations of the color burst signal as might be caused by noise.